Metal diaryl and dialkyl dithiophosphates, especially zinc dithiophosphate, have long been used as antiwear additives and antioxidants in hydraulic oils, motor oils, automatic transmission fluids and the like. Processes for the production of these compounds are also well-known. In the manufacture of such metal dithiophosphates, dithiophosphoric acid is commonly neutralized with a metal base, such as zinc oxide. This neutralization step does not take place readily and commonly a large excess of the base is used; and sometimes in conjunction with a promoter. Promoters used for this purpose have varied widely. U.S. Pat. No. 3,562,306 discloses an inorganic zinc salt may act as a promoter. Ammonia and other amines have also been disclosed to act as promoters see for example U.S. Pat. Nos. 3,573,293; 3,836,745; 4,377,527 and 4,377,527. Acid promoters have also been disclosed, such as strong acid (hydrochloric, perchloric or nitric), sulfonic acids and carboxylic acids, see for example U.S. Pat. Nos. 3,347,790; 3,290,347 and 4,085,053. Where the latter employs an acidic promoter followed by a weak base. Nitrogen dioxide has been suggested as a promoter in U.S. Pat. No. 4,092,341.
As known in the art, promoter use can have undesirable side effects. Use of some promoters can lead to a product which may have stability issues, have relatively high sediments, have difficultly filtering, have a dark color and/or have an unacceptable haze. While certain promoters can improve crude sediment levels, the crude sediment levels remain sufficiently high so that filtration is required. Typically, in conjunction with filtration additional components are used to reduce the sediment and haze to more acceptable levels. However, the use of these additional components has drawbacks. Preferably, process conditions could avoid the need for additional components. Surprisingly, it has been discovered that a process employing a controlled delayed addition of a particular class of promoters during neutralization of a dialkyl dithiophosphoric acid derived from primary alcohol and subsequent overbasing yields a low sediment, low haze, overbased metal dithiophosphate composition.